1. Field of the Invention
The present invention relates to improvements in a shift control system for an automatic transmission, and more particularly, to a shift control system for an automatic transmission, which has a plurality of frictional elements and can shift the transmission by engaging at least a frictional element by increasing the pressure of a hydraulic fluid.
2. Description of the Prior Art
An automatic transmission is so constructed as to determine a power transmission path (shift gear or stage) of a gear shift system by selectively actuating a plurality of frictional elements, such as clutches and brakes, based on the fluid pressure and change the shift by switching the frictional elements to be actuated. It is well known that in an automatic transmission having a plurality of frictional elements, a shift for the transmission can be made by engaging at least a frictional element by increasing the pressure of a hydraulic fluid.
As discussed after, a shift control of the present invention can be adapted for an automatic transmission which has one or more frictional elements to be engaged (the number of engaging-side frictional elements may be equal to or greater than one, and a releasing-side frictional element may not be required or the number of releasing-side frictional elements may be equal to or greater than two), and is not limited to a so-called interchange shift which has a single releasing-side element and a single engaging-side element (also called a 1-to-1 interchange shift). The background of the invention will become apparent from the following description of the exemplified case of automatic transmission. In case of such a shift, an automatic transmission makes its shift through a so-called interchange of frictional elements, in which one frictional element is released by reducing the hydraulic (fluid) pressure while the other frictional element is engaged by increasing the hydraulic fluid pressure.
Note that a frictional element to be switched to a released state from an engaged state at the time of carrying out the interchange shift is called a releasing-side frictional element and its hydraulic fluid pressure to be applied to the frictional element is called a releasing-side hydraulic (fluid) pressure, and that a frictional element to be switched to an engaged state from a released state is called an engaging-side frictional element and its hydraulic fluid pressure to be applied to the frictional element is called an engaging-side hydraulic (fluid) pressure.
In the case of the interchange shift, for example, at the time of executing the interchange shift, the interchange shift can be accomplished under releasing/engaging control in which the engaging-side frictional element is engaged by increasing the engaging-side hydraulic fluid pressure while the releasing side frictional element is being released by reducing the releasing-side hydraulic fluid pressure. As a engaging-side control (for engaging-side frictional element), it is known to independently and sequentially carry out a so-called piston stroke control and a so-called volume control, as disclosed in Japanese Patent Provisional Publication No. 7-286663(Document 1) which is referred hereinafter as a conventional technique. Accordingly, a control needed for the piston stroke and a control needed to secure a volume (of hydraulic fluid to be supplied to the frictional element) are performed independently (it is possible to independently perform learning etc.)
In consideration of the following points, therefore, a shift control for an automatic transmission still has been required to be improved.
(A) Because the progress of the shift can not be controlled on the engaging-side (frictional element) in drive-down (Driv Down) shift, the timing at which the volume is needed is determined irrespective of the volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself. Depending on the situation, therefore, the volume may become necessary while an engaging-side frictional element is doing a piston stroke; however, it is difficult to deal with such a case.
This will be explained with reference to FIG. 8 which shows the relationship between a turbine speed Nt or the number of rotations (per unit time) of the input shaft of the transmission and the volume needed by the engaging-side frictional element. The progressing speed of an inertia phase is determined mainly by the releasing-side hydraulic pressure (the progress of the inertia phase is managed by controlling the releasing-side frictional element). In this case, however, while a volume (or a necessary volume which is required for carrying out the shift) needed by the engaging-side frictional element is also determined by the degree of the progress of the inertia phase, what timing and when the volume becomes necessary cannot be controlled by the engaging-side hydraulic pressure itself. As the timing at which the engaging-side frictional element needs the volume cannot be controlled by the engaging-side frictional element itself, there may occur the aforementioned situation where it is difficult to provide the necessary volume.
(B) Unless the end of the piston stroke is detected by some kind of means, a wasteful time is certainly produced. It is not therefore possible to cope with the shift that demands, for example, a high response (such as the shift made upon a drive-down).
FIG. 9 is a diagram which is also referred to in the later description of an embodiment of the present invention. As apparent from this diagram, linkage between the piston stroke control and the volume control is needed and leads to a wasteful time. (The control to link the two controls causes a lag.) The necessity of such a link control means that it is difficult to secure the necessary volume promptly after the piston stroke ends. The piston stroke control may be carried out using a timer. The use of a timer inevitably requires a certain extra or surplus time, thus producing a wasteful time. (The extra time causes a lag.)
(C) A supplementary explanation of the aforementioned problems will now be given. From the viewpoint of the performances or the like needed for the engaging-side hydraulic pressure for the drive-down shift, the following points are to be considered.
(1) In the latter half of the inertia phase, the volume becomes necessary.
(2) As the degree of the progress of the inertia phase cannot be controlled by the engaging-side frictional element (as discussed referring to FIG. 8, the degree of the progress of the inertia phase is controlled in the drive-down shift mainly by the releasing-side hydraulic pressure), the time when the volume becomes necessary is irrelevant to the status of the engaging-side frictional element.
(3) As the control is used in the drive-down shift, the unnecessary lag is ideally not allowable and the volume is to be increased to the required level as fast as possible.
If shift control depends on the control of the engaging-side frictional element as shown in FIG. 9 even in the case of FIG. 8 where the shift progresses irrespective of the engaging-side volume (or the volume of the hydraulic fluid to be supplied to the engaging-side frictional element), this case is similar to the situation (2) so that the volume may become necessary during piston stroke control but cannot be coped with. Further, because the employment of a timer control (using the timer) to end the piston stroke inevitably produces a wasteful time (see FIG. 9), the timer control is not suitable for the shift that demands a response for the reason given in the paragraph (3).
It is therefore difficult to both surely finish the piston stroke within an intended time even if it is unclear when the volume becomes necessary and promptly secure the necessary volume after the completion of the piston stroke at the same time.
(D) It is desirable that even when the shift progresses irrespective of the engaging-side volume, the completion of the piston stroke and securing of the necessary volume are to be satisfied at the same time. It is also desirable to be able to effectively cope with the shift that demands a high response, surely finish the piston stroke within the intended time and secure the necessary volume immediately upon completion of the piston stroke.
Accordingly, it is an object of the present invention to provide an improved shift control system for an automatic transmission which is improved in consideration of the above-described points as well as points that will be discussed later and which can adequately shift the transmission by engaging at least an engaging-side frictional element by increasing the pressure of a hydraulic fluid.
Another object of the present invention is to provide an improved shift control system for an automatic transmission, which provides an engaging-side (frictional element) control that can effectively cope with a down shift demanding a quick response and other shifts, and can allow the piston stroke to surely be finished within an intended time, while securing a necessary volume of a hydraulic fluid to be supplied to the engaging-side frictional element immediately after completion of the piston stroke of the engaging-side frictional element.
An aspect of the present invention resides in a shift control system for an automatic transmission. The shift control system comprises a plurality of frictional elements including at least an engaging-side frictional element which is able to make a shift for the automatic transmission upon being engaged by increasing a hydraulic pressure of a hydraulic fluid to be supplied to the engaging-side frictional element. Additionally, a controller is provided to be programmed to carry out (a) changing a control for engaging the engaging-side frictional element between in a first condition in which a speed of progression in the shift is managed mainly in accordance with a volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself and in a second condition in which the shift progresses regardless of the volume of hydraulic fluid to be applied to the engaging-side frictional element itself; and (b) accomplishing a volume control for the engaging-side frictional element from a timing immediately after a decision of the shift under a command hydraulic pressure of the hydraulic fluid to be supplied to the engaging-side frictional element in the second condition, the command hydraulic pressure having a lower limit value for securing that a piston stroke of the engaging-side frictional element completes within a predetermined time.
Another aspect of the present invention resides in a shift control system for an automatic transmission. The shift control system comprises a plurality of frictional elements including at least an engaging-side frictional element which is able to make a shift for the automatic transmission upon being engaged by increasing a hydraulic pressure of a hydraulic fluid to be supplied to the engaging-side frictional element. Additionally, a controller is provided to be programmed to carry out (a) determining that the automatic transmission is in a first condition in which a speed of progression in the shift is managed mainly in accordance with a volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself; (b) determining that the automatic transmission is in which the shift progresses regardless of the volume of hydraulic fluid to be applied to the engaging-side frictional element itself; (c) applying a first control and a second control for engaging the engaging-side frictional element respectively in the first condition and the second condition, the first and second controls being different from each other and changed over from one to the other; (d) setting a lower limit value of a command hydraulic pressure of the hydraulic fluid to be supplied to the engaging-side frictional element, the engaging-side frictional element is engaged under the command hydraulic pressure, the lower limit value securing that a piston stroke of the engaging-side frictional element completes within a predetermined time; and (e) accomplishing a volume control for the engaging-side frictional element from a timing immediately after a decision of the shift under the command hydraulic pressure of the hydraulic fluid in the second condition.
According to the invention, the automatic transmission has a plurality of frictional elements and can shift the transmission by engaging at least an engaging-side frictional element by increasing the pressure of a hydraulic fluid, and the shift control system switches a control for the engaging-side frictional element between a first case or condition where the progressing speed of shift is controlled mainly by the volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself and a second case or condition where the shift progresses irrespective of the volume of the engaging-side frictional element itself. In the second case, the shift control system executes the control on the engaging-side frictional element in such a way as to perform the volume control immediately after the shift is determined, while taking a command pressure for guaranteeing the completion of the piston stroke within a predetermined time as a lower limit value.
It is therefore possible to properly change the control for the engaging-side frictional element between the first and second cases, thus ensuring a delicate control to improve the applicability, and to guarantee the completion of the piston stroke within a predetermined time and promptly increase the hydraulic fluid pressure to or above the hydraulic pressure for the necessary volume upon completion of the piston stroke at the same time. The increase in hydraulic fluid pressure can realize a shift control that can suitably carry out the shift by engaging the at least an engaging-side frictional element. Note that the invention is not limited to the interchange shift which involves a single release element and a single engaging element. The invention is adaptable if there is at least an engaging-side frictional element to be engaged and is in no way limited to the 1-to-1 interchange shift. (The number of engaging-side frictional elements may be equal to or greater than one, and a releasing-side frictional element may not be required or the number of releasing-side frictional elements may be equal to or greater than two.) The invention is neither limited to a down shifting. The invention can be adapted to the drive up (Driv UP) and coast down (Coast Down) in the first case, and to the coast up (Coast UP) and drive down (Driv Down) in the second case. Because the hydraulic pressure that guarantees the completion of the piston stroke within a predetermined time can be set as a lower limit value at the time of drive-down shift in an embodiment of the invention that will be discussed later, for example, the piston stroke can surely be finished within the intended time. It is also possible to always perform volume control and secure the necessary volume immediately upon completion of the piston stroke.
Demerits or the like of the control method for the engaging-side frictional elements that is employed for the shift in the second case will be additionally discussed below.
The method of executing the volume control from the beginning by using the hydraulic pressure that guarantees the completion of the piston stroke within a predetermined time as a lower limit value has a merit that the necessary volume can be secured immediately upon completion of the piston stroke. By way of contrast, in an automatic transmission of a directly valve-actuating type that does not have a buffer element (accumulator or the like) in the hydraulic circuit, the slop along which the volume increases cannot be controlled thereby causing the volume to be reached the necessary volume at once, thus causing a sudden variation in the torque of the output shaft. Accordingly, even when the necessary volume is small and is reached at once, therefore, the control according to the present invention can be used without causing the ill effect to appear, only when a variation in the then volume is small.
In the first case, because the engaging-side frictional element will cause the shift to progress against the engine torque, the necessary volume during the inertia phase is relatively large. However, in the second case, as the progress of shift is made by the engine torque itself, the volume necessary during the inertia phase is relatively small.
In view of the above, the second case is more suitable as the condition under which the method of executing the volume control from the beginning by using the hydraulic pressure that guarantees the completion of the piston stroke within a predetermined time as a lower limit value is carried out without causing the ill effect.
The present invention is preferably provided with an arrangement in which changing the control is made in accordance with a first state in which a turbine torque of a torque converter is in a drive mode and a second state in which the turbine torque is in a coast mode. With this arrangement, the invention can be embodied adequately and can accomplish the above-described control. In case that the progressing speed of shift is controlled mainly by the volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself, a control switching can be made depending on whether the turbine torque is in drive mode or coast mode only when the shift progresses regardless of the volume of the engaging-side frictional element. It is therefore possible to adequately accomplish the above-described shift by properly changing the control on the engaging-side frictional element in accordance with the turbine torque based switching.
Additionally, the present invention is preferably provided with an arrangement in which a piston stroke control is made before the volume control in the first condition, the piston stroke control being for the engaging-side frictional element and accomplished under a command hydraulic pressure of the hydraulic fluid to be supplied to the engaging-side frictional element. With this arrangement, the invention can be embodied adequately and can accomplish the above-described control. Under this situation, in the first case where the progressing speed of shift is controlled mainly by the volume of the hydraulic fluid to be supplied to the engaging-side frictional element itself, the control on the engaging-side frictional element can be executed in the order of piston stroke control and volume control and the invention can carry out the control on the engaging-side frictional element as the one having such contents.
Further, the present invention is preferably provided with an arrangement in which the controller is programmed to further carry out accomplishing a volume control for the engaging-side frictional element under a command hydraulic pressure of the hydraulic fluid to be supplied to the engaging-side frictional element in the second condition. The command hydraulic pressure has a lower limit value for securing that a piston stroke state of the engaging-side frictional element is maintained in the volume control so as to secure maintenance of the piston stroke state even if a necessary volume of the hydraulic fluid to be supplied to the engaging-side frictional element is reduced. With this arrangement, the invention can be embodied adequately and can accomplish the above-described control. Under this situation, even in the first case, a lower limit value which ensures that the piston stroke state is maintained is so provided during the volume control as to be able to control the shift in such a manner that even when the necessary volume becomes smaller, the piston stroke state is guaranteed. Even when the necessary volume decreases during volume control, therefore, it is possible to ensure that the piston stroke which has been completed once does not return.